The following meanings for the abbreviations used in this specification apply:
C-RNTI Cell Radio Network Temporary Identifier
eNB enhanced Node-B
E-UTRA Evolved Universal Terrestrial Radio Access.
GPRS General Packet Radio Service
HetNet Heterogeneous networks
HO Handover
HOF Handover failure
ID Identifier
LTE Long term evolution
LTE-A LTE-Advanced
MLB Mobility load balancing
MRO Mobility robustness optimization
NRT Non real time
QoS Quality of Service
RLF Radio link failure
RNTI Radio network temporary identifier
RRC Radio resource control
RT Real time
SON Self organized network
TS Traffic steering
UE User equipment
UL Uplink
Embodiments of the present invention relate to LTE and LTE-A, and in more detail to SON MRO (Mobility Robustness Optimisation) mechanism, which was first developed in Rel.9 LTE system. It was based on RRC re-establishment procedure and inter-eNB notifications: when one eNB detected a re-establishment attempt, it informed about the eNB where the failure happened about this re-establishment attempt. That eNB, based on the information provided in the notification, could discover the reason of the failure (and possibly further notify the eNB where the problem started) based on still available UE context and possibly additional measurements provided from the UE.
In Rel.10 this mechanism was changed: since network information is not always precise (HO command may fail in delivery) and not complete if the UE context is missing (which is the case if the re-establishment fails and the C-RNTI is lost), the Rel.10 MRO solution was based on the information reported from the UE. This is very precise if certain assumptions are made concerning the situations: that all the HOs are triggered only because of radio reasons and the triggering criteria are the same for all the UEs. Those assumptions are needed, because the UE is not aware of the HO process at the eNB and receives only the HO command, therefore can not report back anything that lead to the handover decision in the eNB.
In Re1.11, the MRO-MLB (or MRO—traffic steering, in general) coordination is still considered as a possible focus area. Since activity of MLB may trigger HOs that are not related to radio conditions (actually, usually against radio conditions, which may lead to failures), the assumptions made in Rel.10 may not hold any longer. Also, different criteria may be used in HetNet deployment, where pico cells may be used to handle particular types of UEs and therefore HO triggering to those pico cells may be different than those to other macro cells.
Thus, the Rel.10 MRO solution is based on the UE report, which, while providing detailed radio conditions of the failure, does not tell anything about the triggering condition at the source eNB. If it can be assumed all the HOs are carried out because of radio conditions (mobility) and that all users are treated in the same way (the same trigger point), this could be sufficient. However, in a modern network, in presence of other SON features, this is too general, though.
In the following, two examples for illustrating the problem are described.